While being glass, crystallized glass has secondary optical non-linear properties inherent to crystal, and thus, it has a wide transmitting wavelength region, and it can be easily connected to glass optical fiber. Thus, it is expected as a new photonics material for an optical integrated circuit or an optical switch for light wave control.
On the other hand, an optical component such as an integrated optical switch using a non-linear optical single crystal such as LiNbO3 to utilize the secondary optical non-linear properties inherent to the crystal, has been proposed, but it is difficult to connect it to glass optical fiber. Besides, preparation of the crystal material is difficult, and the shape-forming property is also inferior to a glass material, whereby it is very difficult to process it into a desired shape.
Whereas, a glass material has a characteristic such that the shape can be controlled simply, easily and inexpensively, for example, in forming into a thin film or in drawing it into a fiber. However, it does not essentially have a structure having an electrical polarity aligned (oriented) in a specific direction as in the case of a crystal material, and in principle, glass does not show secondary optical non-linear properties. Accordingly, it has not heretofore been realized to use glass as an active light wave control functional material, as expected by crystal, for e.g. an optical switch, from the viewpoint of the degree of such secondary optical non-linear properties.
In or after 1990, a method for preparing crystallized glass by using pulsed laser was reported. For example, a method for producing crystallized glass was reported wherein microcrystals were selectively precipitated in the inside of a glass (Patent Documents 1 and 2). However, in such prior art, it was not possible to control the alignment of crystal particles, and it was not possible to sufficiently obtain secondary optical non-linear properties inherent to a crystal material.
As a method for preparing crystallized glass by utilizing heat by a laser, it is known to irradiate a glass surface with a CO2 laser to induce crystallization at the glass surface (Non-Patent Document 1). Further, a method for preparing a crystallized optical waveguide by using a CO2 laser has been reported by U.S. Corning (Patent Document 3). However, in the preparation of crystallized glass by using a CO2 laser, the laser beam is absorbed only by the surface, whereby the crystallization is limited to the glass surface, and the inside can not be processed. Further, the wavelength is a long wavelength, whereby microregions can hardly be processed.
Further, a method for producing crystallized glass made of non-linear optical crystals has been reported which comprises irradiating a bismuth type glass containing samarium with a near infrared laser of a continuous oscillation type with a wavelength of 1,064 nm (Patent Document 4). This method utilizes a phenomenon such that an Nd:YAG laser beam excites transition between the energy levels corresponding to the infrared laser beam, of samarium atoms present in the glass (transition from the energy level of 6H5/2 to the energy level of 6F9/2), and the photoexcited electrons undergo relaxation without radiation (radiation-free relaxation), i.e. efficiently emit heat, so that local heating takes place around samarium atoms. In this connection, it has been proposed to prepare crystallized glass having orientation of crystals aligned by continuously moving the focal position of a laser beam which can be constantly oscillated (Non-Patent Document 2).
However, in the above-mentioned Patent Document 4, with a laser beam having a wavelength of 1,064 nm, crystallization will no develop unless the content of Sm2O3 is at least 3 mol %, when the irradiation power of the laser beam is 100 W/cm2. Therefore, such a method was not applicable to a case of glass components whereby samarium can not be incorporated in a large amount to the glass or to a case of glass components whereby the temperature for precipitation of the desired group of crystal particles is high and the crystallization is difficult solely by the heat generation by the laser. Therefore, it is desired to incorporate ion species which are capable of converting the irradiated laser beam to heat efficiently with its content smaller than samarium, but there has been no report on such prior art.
Patent Document 1: JP-A-11-71139
Patent Document 2: JP-A-2005-132693
Patent Document 3: JP-A-2004-523917
Patent Document 4: JP-A-2003-98S63
Non-Patent Document 1: C. Mai, Supplement Riv. Staz. Sper. Vetro XXIII (1993) 435, Adelaine F. Maciente et al., Journal of Non-Crystalline Solids 306 (2002) 309-312
Non Patent Document 2: T. Honma et al., Applied Physics Letters, vol. 83, no. 14, pp. 2796-2798, 2003